Distributed sensing is commonly used to determine the conditions of a monitored object by collecting data from a distributed set of points on or in the vicinity of the object. The technique is widely used for example for structural health monitoring (SHM), where a structure such as a bridge, a building or an airplane, is monitored substantially simultaneously at the distributed set of points, to detect a structural fault at an early stage of the fault progress.
Some methods for distributed sensing utilize a deployed optical fiber, which is in mechanical or thermal contact, substantially along at least a portion of the fiber's length, with the object that needs to be monitored. Optical fibers, typically made as flexible transparent thin fibers, are commonly used for highly efficient transportation of light signals over large distances. Within some such methods for distributed sensing, each point, area, or segment of the optical fiber may be used for sensing; light signals, indicating sensed data, are transmitted through the fiber to be collected and processed at any one or both of the optical fiber's ends.
Stimulated Brillouin scattering (SBS) in optical fibers is an underlying effect employed by several methods for distributed sensing of local strain and temperature variations, over distances that may reach tens of kilometers. Techniques that employ SBS are for example Brillouin Optical Time Domain Reflectometry (BOTDR), Brillouin Optical Time Domain Analysis (BOTDA) and Brillouin Optical correlation Domain Analysis (BOCDA). Within these techniques, an intense light wave (a pump wave)—that may be amplitude-modulated, frequency-modulated or phase-modulated—is transmitted to the fiber, while a scattered light wave, generally having a frequency shifted from that of the pump wave, is received from the optical fiber, monitored and analyzed in the time and frequency domains.
Stimulated Brillouin scattering (SBS) is a nonlinear optical propagation effect, in which an optical wave (a pump wave) propagating forward in an optical medium, is scattered inelastically by phonons of the medium. The phonons are acoustic waves, generated in the medium by the propagating pump light wave, through electrostriction.
When SBS occurs in an optical fiber, the pump light wave may be backscattered by the phonons in the optical fiber that are generated as described above, thereby generating a backscattered light wave propagating in the opposite direction to the pump light wave. The frequency of the backscattered wave is different from the frequency of the pump wave by the Brillouin frequency shift ΩB, which is generally on the order of magnitude of 10 GHz in standard optical fibers. The ΩB frequency is a property of the optical medium in which the waves propagate, but it may vary with environmental conditions such as temperature and mechanical strain. Further, when SBS is generated in an optical fiber, and a second optical wave (a probe wave) is transmitted into the fiber in the opposite direction to the pump wave, the probe wave may be forward-scattered by the phonons in the optical fiber that are generated as described above. The forward-scattered probe light wave may further be amplified considerably if its frequency is set to be substantially equal to that of the backscattered wave, namely if its frequency is shifted from that of the pump light wave by exactly ΩB.
In BOTDR, a pump wave of frequency ω0 is pulse-modulated, and the light pulses are transmitted to the fiber. A backscattered wave, having a frequency of approximately ω0−ΩB, is then generated in the fiber due to SBS along the fiber's length. The backscattered wave is detected, and its frequency is instantaneously monitored as a function of time, following each transmitted pulse. If the fiber is exposed to uniform environmental conditions throughout its length, the detected wave frequency is constant, and shifted from that of the transmitted pulses by ΩB. If, however, the fiber is exposed, in some portion thereof, to some different conditions, e.g., strain different from that of other portions of the fiber, then the backscattered wave from that portion has frequency that deviates by some difference ΔΩ from ω0−ΩB. The magnitude of the frequency difference ΔΩ is substantially proportional to the magnitude of the strain (or temperature) variation, typically by 0.5 GHz/% strain, and 1 MHz/° K. Further, the time gap Δt between each transmitted pulse and the time when such a frequency change is detected with the backscattered wave, indicates the location, along the optical fiber, where the strain is applied: denoting the light wave group velocity in the fiber by vg, the distance, along the fiber, from the end of the fiber to that location is ½Δt·vg. Thus, by mapping the magnitude of the backscattered wave as a function of both time and frequency, the position and magnitude of strain (and temperature) variance is detected.
In Brillouin optical time domain analysis (BOTDA), a pump light wave is transmitted from one end of the fiber, while counter-propagating probe waves, typically substantially weaker than the pump waves, are transmitted from the other end of the optical fiber. The pump wave amplifies the probe waves having the frequencies that match the local Brillouin frequency shift in the fiber. The position-dependent frequency shift of the probe wave is then detected in a similar manner to the description relating to BOTDR above, through mapping the magnitude of the amplified probe as a function of both time and frequency.
In Brillouin optical correlation-domain analysis (BOCDA), a continuous, constant-magnitude pump wave is transmitted into an optical fiber from one end, and a continuous, constant-magnitude probe wave is transmitted into the optical fiber from the opposite end, and in the opposite direction to the pump wave. The frequencies of the two waves, which are nominally ΩB apart, are synchronously modulated by a common sine wave. Due to the modulation, the frequency difference between the two counter-propagating waves remains stationary only at particular fiber locations, known as correlation peaks, whereas the frequency difference elsewhere is oscillating. Consequently, effective SBS amplification, and hence localized measurement, is restricted to the location, along the fiber, where correlation peaks occur.